Low thermal expansion resin material and composite shaped article

ABSTRACT

A resin material comprising a polyamide having as chemical structural unit at least one aromatic ring which can rotate around its molecular axis but has no flexibility at another direction, said polyimide being oriented at least at a uniaxial direction, has a low thermal expansion coefficient and can be shaped together with an inorganic material into one body to give a composite shaped article.

This is a division of application Ser. No. 636,736, filed Aug. 1, 1984,now U.S. Pat. No. 4,690,999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a low thermal expansion resin materialcomprising a polyimide having a special chemical structure and beingoriented and a composite shaped article using the same.

2. Description of the Prior Art

Organic polymers have by far larger coefficients of thermal expansion(coefficient of linear expansion), for example 4×10⁻⁵ K⁻¹ or larger inmost cases, even at a temperature range below the glass transitiontemperature compared with metals and inorganic materials. Problemscaused by such large coefficients of linear expansion of organicmaterials are remarkably numerous. So that, it is not too much to saythat all the reasons for not progressing as desired in applications anddevelopment of organic polymers are based on this. For example, in aflexible printed circuit (FPC) comprising a film and a conductor, thereis desired a film obtained by coating or hot pressing a flexible filmmaterial on a metal foil. But since it is necessary to cure and dry at ahigh temperature after the coating or to hot press, there arises aproblem of curling due to thermal stress caused by difference incoefficients of thermal expansion after cooled to room temperature.Usually, in order to avoid such a problem, the film and the conductorare laminated by using an adhesive which can be cured at lowtemperatures. But in the case of FPC which is required to have heatresistance, since the adhesive curable at low temperatures is generallypoor in heat resistance, heat resistance inherently having cannot beexhibited even if a heat resistant film such as a polyimide film is usedas substrate. On the other hand, in the case of coating, when an organicpolymer is coated on a metal plate or inorganic material having a verysmall coefficient of thermal expansion compared with the organicpolymer, there take place deformation, cracks of the film, peeling off,of the film, breaking of substrate due to thermal stress caused bydifferences in coefficients of thermal expansion. For example, when acoating film is formed on a silicon wafer as a protective film for LSI(large-scale integrated circuit) or IC (integrated circuit), the waferis warped, which results in making photolithography for patterningimpossible, making resolving power extremely worse, or in the case oflarge thermal stress, peeling a passivation film, or sometimes causingcleavage and breaking of silicon wafer per se.

As mentioned above, there are a large number of problems caused by thelarge coefficient of linear expansion of organic polymers and there havelong been desired organic polymers having a low thermal expansioncoefficient.

Under such circumstances, the present inventors have studied in detailthe relationship between chemical structure and the thermal expansioncoefficient by using many heat resistant resin materials, particularlyby using various polyimides synthesized. There have been providedvarious kinds of polyimides, but practically synthesized or used onesare very small. Heretofore, practically synthesized, reported ormarketed polyimides are those obtained by using as raw materials anaromatic diamine such as diaminodiphenyl ether, diaminodiphenylmethane,para-phenylenediamine or diaminodiphenyl sulfide, and a tetracarboxylicdianhydride such as pyromellitic dianhydride,benzophenonetetracarboxylic dianhydride, tetracarboxydiphenyl etherdianhydride, or butanetetracarboxylic dianhydride. But these polyimideshave a remarkably large coefficient of linear expansion of 4 to 6×10⁻⁵K⁻¹.

But the present inventors have found that polyimides having an extremelysmall linear expansion coefficient and remarkably excellent tensilestrength compared with those mentioned above can be obtained from aspecial aromatic diamine and a tetracarboxylic dianhydride mentionedbelow. This invention is based on such a finding.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a low thermal expansionresin material having an extremely small thermal expansion coefficientcorresponding to that of an inorganic material such as a metal, ceramicor glassy material as well as excellent mechanical properties, and acomposite shaped article using the same.

This invention provides a low thermal expansion resin materialcomprising a polyimide having as chemical structural unit at least onearomatic ring which can rotate around its molecular axis but has noflexibility at another direction, said polyimide being oriented at leastat an uniaxial direction.

This invention also provides a composite shaped article comprising aninorganic material and a low thermal expansion resin material shapedinto one body, said resin material comprising a polyimide having aschemical structural unit at least one aromatic ring which can rotatearound its molecular axis but has no flexibility at another direction,said polyimide being oriented at least at a uniaxial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing coefficients of thermal expansion of variousmaterials.

FIG. 2 is a cross-sectional view of a flexible printed circuit obtainedby direct coating.

FIG. 3 is a cross-sectional view of LSI having a multi-layer wiringstructure.

FIG. 4 is a cross-sectional view of a memory element having an α-raysshielding layer.

FIG. 5 is a cross-sectional view of a metal core wiring board mounting afilm-carrier type LSI.

FIG. 6 is a cross-sectional view of a metal-plate-based printed circuitmounting a LSI bonded by lead wire.

FIG. 7 is a graph showing properties of the polyimides of thisinvention.

FIG. 8 is a graph showing the behavior of thermal expansion for No. D1.

FIG. 9 is a graph showing the behavior of thermal expansion for No. A2.

FIG. 10 is a graph showing the behavior of thermal expansion for No. A3.

FIG. 11 is a graph showing the behavior of thermal expansion for No. F3.

FIG. 12 is graphs showing coefficients of thermal expansion forpolyimides.

FIG. 13 is diffractograms for bifix-cured polyimides.

FIG. 14 is diffractograms for A3 films.

FIG. 15 is drawings showing conformations of polyimides.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The chemical structure as main chain of the low thermal expansionpolyimide of this invention includes, for example, as follows: ##STR1##wherein R is an alkyl group, a fluorinated alkyl group, an alkoxy group,a fluorinated alkoxy group, an acyl group or a halogen; l is zero or aninteger of 1 to 4; m is zero or an integer of 1 to 2; and n is zero oran integer of 1 to 3. The low thermal expansion resin material of thisinvention has lower thermal expansion, higher strength, higherelasticity and higher heat resistance than other polymers, even if themolecular arrangement of the resin material is random. When the resinmaterial of this invention is subjected to a molecular orientationtreatment, these properties are by far improved.

Generally speaking, aromatic polymers are rigid but disadvantageouslybrittle. In order to make the whole polymer flexible, there isintroduced a flexible bond such as --O--, --S--, --(CH₂)_(p) -- (p is aninteger of 1 or more), ##STR2## or the like thereinto. Alternatively,when the bonding position of aromatic rings takes at an O-- orm-position, the whole polymer becomes flexible. The same things can beapplied to polyimides. Polyimides now industrially produced havebondings selected from the above-mentioned ones. Therefore, polyimideshaving low thermal expansion as claimed in this invention have not beenknown.

The low thermal expansion polyimides of this invention includes thefollowing repeating units: ##STR3## wherein R, l, m and n are asmentioned above; and k is zero or an integer of 1 to 3.

FIG. 1 is a graph showing thermal expansion coefficients of variousmaterials. As shown in FIG. 1, general organic polymers have largerthermal expansion coefficients then metals and ceramics, but theoriented polyimide of this invention has a clearly smaller thermalexpansion coefficient compared with the general organic polymers. Thatthe thermal expansion coefficient is as small as those of inorganicmaterials such as metals and ceramics means that when the resin materialis used in combination with these inorganic materials, there take placeno thermal stress nor warfage due to the same dimensional change upontemperature changes: this is very important from an industrial point ofview. It is not too much to say that the most important defect ofconventional organic materials is that the thermal expansion coefficientof the organic materials are by far larger than that of the inorganicmaterials.

The low thermal expansion polyimide of this invention can be produced bypolymerization of an aromatic aminodicarboxylic acid derivative, or areaction of an aromatic diamine or an aromatic diisocyanate with anaromatic tetracarboxylic acid derivative. The derivative oftetracarboxylic acid means an ester, an acid anhydride, an acidchloride, or the like. Considering the synthesis process, the use of anacid anhydride is preferred.

The polymerization can generally be carried out at 0° to 200° C. in asolvent such as N-methylpyrrolidone (NMP), dimethyl formamide (DMF),dimethyl acetamide (DMAC), dimethyl sulfoxide (DMSO), dimethyl sulfate,sulfolane, butyrolactone, cresol, phenol, halogenated phenol,cyclohexane, dioxane, etc., alone or as a mixture thereof.

Examples of the aromatic aminodicarboxylic acid derivative are4-aminophthalic acid, 4-amino-5-methylphthalic acid,4-(p-anilino)-phthalic acid, 4-(3,5-dimethyl-4-anilino)-phthalic acid,esters thereof, acid anhydrides thereof, acid chlorides thereof, etc.These aminodicarboxylic acid derivatives can be used alone or as amixture thereof.

Examples of the aromatic diamine are p-phenylenediamine,2,5-diaminotoluene, 2,5-diaminoxylene, diaminodurene,2,3,5,6-tetramethyl-p-phenylenediamine, 2,5-diaminobenzotrifluoride,2,5-diaminoanisole, 2,5-diaminoacetophenone, 2,5-diaminobenzophenone,2,5-diaminodiphenyl, 2,5-diaminofluorobenzene, benzidine, o-tolidine,m-tolidine, 3,3',5,5'-tetramethylbenzidine, 3,3'-dimethoxybenzidine,3,3'-di(trifluoromethyl)benzidine, 3,3'-diacetylbenzidine,3,3'-difluorobenzidine, octafluorobenzidine, 4,4"-diaminoterphenyl,4,4"'-diaminoquataphenyl, etc. These diamines can be used alone or as amixture thereof. It can also be possible to use diisocyanate compoundsof these aromatic diamines.

Examples of the aromatic tetracarboxylic acid derivatives arepyromellitic acid, methylpyromellitic acid, dimethylpyromellitic acid,di(trifluoromethyl)pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylicacid, 5,5'-dimethyl-3,3',4,4'-biphenyltetracarboxylic acid,p-(3,4-dicarboxyphenyl)benzene, esters thereof, acid anhydrides thereof,acid chlorides thereof, etc. These tetracarboxylic acid derivatives canbe used alone or as a mixture thereof.

The polyimide of this invention is, even if not oriented, lower inthermal expansion, higher in strength, and higher in elasticity, thanother polymers, but when the molecular chain is oriented, theseproperties are by far highly enhanced. For example, the molecular chaincan be oriented by stretching a film-like artic le uniaxially orbiaxially. It is known that an aromatic polyimide generally becomeslower in thermal expansion, higher in elasticity and higher in strengthby an orientation treatment, but such effects are by far small comparedwith the effects in this invention. The orientation of polymer chain canbe effected by not only stretching of a film using a stretcher but alsoapplying shrinkage caused by a curing reaction of polyimide or during aprocedure from a precursor varnish to polyimide shape article orshrinkage caused by evaporation of the solvent. For example, bypreventing the shrinkage at the time of curing of a varnish coated, theorientation of molecular chain becomes possible. The stretching amountby curing shrinkage is very small compared with a conventionalstretching method, but even such an orientation treatment issufficiently effective in the polyimide of this invention. In the caseof stretching a film-like shaped article, when a completely curedmaterial to be stretched, the stretching should be conducted at a veryhigh temperature because of extremely high glass transition temperatureof polyimide, and even if stretched at such a high temperature, theorientation is very difficult due to very strong intermolecular cohesiveforce. In order to orient easily, it is preferable to stretch at a stateof containing a solvent to some extent or at a state of polyamide-acidhaving a low glass transition temperature to orient the molecule,followed by complete curing.

That the polyimide of this invention is very low in thermal expansion isquite different from the common knowledge of conventional polymers andthus cannot be expected from prior art. Reasons for this seem to be thatthe polyimide of this invention has an almost linear structure as shownin FIG. 15 (I) and (II).

The polyimide of this invention is very low in thermal expansion but insome cases slightly brittle. In such a case, the brittleness of thepolyimide of this invention can be improved by blending orcopolymerization with other one or more polymers without remarkablyincreasing the thermal expansion coefficient. It is also possible toadjust the thermal expansion coefficient of the polyimide of thisinvention by blending or copolymerization with other one or morepolymers considering the thermal expansion coefficient of inorganicmaterial in the case of forming a composite shaped article. From theviewpoint of heat resistance, the use of polyimide as other polymers ispreferred. Such a polyimide can be obtained from a diamine and atetracarboxylic acid or a derivative thereof mentioned below.

Examples of such a diamine are m-phenylenediamine, 4,4'-diaminodiphenylmethane, 1,2-bis(anilino)ethane, 4,4'-diaminodiphenyl ether,diaminodiphenyl sulfone, 2,2-bis(p-aminophenyl)propane,2,2-bis(p-aminophenyl)hexafluoropropane,3,3'-dimethyl-4,4'-diaminodiphenyl ether,3,3'-dimethyl-4,4'-diaminodiphenylmethane, diaminotoluene,diaminobenzotrifluoride, 1,4-bis(p-aminophenoxy)benzene,4,4'-bis(p-aminophenoxy)biphenyl,2,2-bis{4-(p-aminophenoxy)phenyl}propane, diaminoanthraquinone,4,4'-bis(3-aminophenoxyphenyl)diphenyl sulfone,1,3-bis(anilino)hexafluoropropane, 1,4-bis(anilino)octafluorobutane,1,5-bis(anilino)decafluoropentane,1,7-bis(anilino)tetradecafluoroheptane, diaminosiloxanes represented bythe formula: ##STR4## or by the formula: ##STR5## wherein R₅ and R₇ areindependently a divalent organic group such as alkylene, phenylene,substituted phenylene, etc.; R₄ and R₆ are independently a monovalentorganic group such as alkyl, phenyl, substituted phenyl, etc.; and p andq are independently integers of 1 or more,2,2-bis{4-(p-aminophenoxy)phenyl}hexafluoropropane,2,2-bis{4-(3-aminophenoxy)phenyl}hexafluoropropane,2,2-bis{4-(2-aminophenoxy)phenyl}hexafluoropropane,2,2-bis{(4-(4-aminophenoxy)-3,5-dimethylphenyl}hexafluoropropane,2,2-bis{4-(4-aminophenoxy)-3,5-bis(trifluoromethyl)phenyl}hexafluoropropane,p-bis(4-amino-2-trifluoromethylphenoxy)benzene,4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl,4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl,4,4'-bis(4-amino-2-trifluoromethylphenoxy)diphenyl sulfone,4,4'-bis(3-amino-5-trifluoromethylphenoxy)diphenyl sulfone,2,2-bis{4-(4-amino-3-trifluoromethylphenoxy)phenyl}hexafluoropropane andthe like. It is also possible to use aromatic diisocyanates obtained byreacting these diamines with phosgene such as tolylene diisocyanate,diphenylmethane diisocyanate, naphthalene diisocyanate, diphenyl etherdiisocyanate, phenylene-1,3-diisocyanate, etc. These diamines anddiisocyanates can be used alone or as a mixture thereof.

Examples of such a tetracarboxylic acid or a derivative thereof are2,3,3',4'-tetracarboxydiphenyl, 3,3',4,4'-tetracarboxydiphenyl ether,2,3,3',4'-tetracarboxydiphenyl ether,3,3',4,4'-tetracarboxybenzophenone, 2,3,3',4'-tetracarboxybenzophenone,2,3,6,7-tetracarboxynaphthalene, 1,4,5,7-tetracarboxynaphthalene,1,2,5,6-tetracarboxynaphthalene, 3,3',4,4'-tetracarboxydiphenylmethane,2,2-bis(3,4-dicarboxyphenyl)propane,2,2-bis-(3,4-dicarboxyphenyl)hexafluoropropane,3,3',4,4'-tetracarboxydiphenyl sulfone, 3,4,9,10-tetracarboxyphenylene,2,2-bis{4-(3,4-dicarboxyphenoxy)phenyl}propane,2,2-bis{4-(3,4-dicarboxyphenoxy)phenyl}hexafluoropropane,butanetetracarboxylic acid, cyclopentanetetracarboxylic acid, estersthereof, acid anhydrides thereof, acid chlorides thereof, etc. Thesetetracarboxylic acids or derivatives thereof can be used alone or as amixture thereof.

The polyimide of this invention can be modified with a compound havingone or more reactive functional groups or by introducing a crosslinkedstructure or ladder structure thereinto, as mentioned below.

(i) Modification with a compound of the formula (X) to introduce one ormore pyrrolone rings or isoindoloquinazolinedione rings. ##STR6##wherein R¹ is an aromatic organic group having a valance of 2+x; Z is aNH₂ group, CONH₂ group or SO₂ NH₂ group and is positioned at an orthoposition to the amino groups; and x is 1 or 2.

(ii) Modification with an amine, diamine, dicarboxylic acid,tricarboxylic acid, tetracarboxylic acid, or a derivative of thesecompounds, these compounds having at least one polymerizable unsaturatedbond, to form a crosslinked structure at the time of curing. Examples ofthese unsaturated compounds are maleic acid, nadic acid,tetrahydrophthalic acid, ethynylaniline, and derivatives thereof.

(iii) Modification with an aromatic amine having a phenolic hydroxylgroup or a carboxyl group to form a cross-linked structure by using acrosslinking agent reactive with the phenolic hydroxyl group or carboxylgroup.

In order to shape the low thermal expansion polyimide and an inorganicmaterial into one body to form a composite shaped article, adhesivenessis important in this invention. As the inorganic material, there can beused metals such as Cu, Cr, Au, Ag, Ni, Al, Fe, Co, Zn, Pb, Sn, Ti, Mo,Pd, etc., alloys thereof, Si, SiC, SiO₂, Si₃ N₄, Al₂ O₃, BN and the likeceramics, Fe₂ O₃, semiconductors such as Ga.As, glass, etc. in the formof film, plate, powder, fiber, etc. In order pertain strongadhesiveness, it is preferable to roughen the surface of inorganicmaterial, or to subject the inorganic material to a surface treatmentwith a silane coupling agent, a titanate coupling agent, aluminumalcholate, aluminum chelate, zironium chelate, aluminum acetylacetone,etc. These surface treating agents can be added to the low thermalexpansion polyamide. The adhesiveness of the polyimide to inorgaicmaterials can be improved by modifying the polyimide with a siloxanecontaining diamine, or a tetracarboxylic acid or a derivative thereof.

The slow thermal expansion resin material of this invention may includeone or more inorganic powders, organic powders, metal powders, fibers,chopped strands in order to further lower the thermal expansioncoefficient, to enhance elasticity or to control fluidity.

The composite shaped article obtained by shaping the low thermalexpansion resin material and the inorganic material into one body can beused as follows:

(1) IC or LSI having a carrier film

(2) a flat cable

(3) a flexible printed circuit

(4) LSI having a wiring insulating film

(5) LSI having a moisture resistant protective film

(6) LSI having an α-rays shielding film

(7) a film insulating coil

(8) a semiconductor having a passiviation film

(9) a metal core printed board having a polyimide insulating film

The low thermal expansion resin material of this invention can be usedas follows.

(10) a solar cell

(11) organic fibers

(12) FRP (fiber reinforced plastic) reinforced with low thermalexpansion polyimide fibers

(13) an organic fiber

The composite shaped articles of this invention obtained by shaping thelow thermal expansion resin material and the inorganic material into onebody will be explained referring to the drawings.

FIG. 2 is a cross-sectional view of a flexible printed circuit obtainedby coating a varnish of precursor of the low thermal expansion polyimide2 directly on a copper foil 1, curing the resin while preventing theshrinkage by curing, followed by patterning of the copper foil. Sincethe polyimide has low thermal expansion, there does not take placecurling due to differences in thermal expansion coefficients even cooledto room temperature after cured. As a result, there can be obtained aflat flexible printed circuit. In the case of conventional FPC, since anadhesive is used, heat resistance is largely lowered. In contrast, inthis invention, there is no such lowering in heat resistance and theadhesive strength is very strong.

FIG. 3 is a cross-sectional view of LSI having a multi-layer wiringstructure. Numeral 3 denotes a silicon wafer, 4 a SiO₂ film, 5 Al(aluminum) wiring, and 6 an insulating thin film of the low thermalexpansion polyimide. When the insulating thin film is formed by a spincoating method, level differences of aluminum wiring can remarkably belessened to give a flat and highly reliable wiring structure. Furthersince the thermal expansion is very low, stress to the element is verysmall. when the thermal stress is large, there arise cracks in theelement. Further, the low thermal expansion resin material of thisinvention can easily be processed. The etching speed is about 2 times asfast as that of a conventional polyimide. In FIG. 3, when the surfacelayer 6 is made of an inorganic film such as SiO₂ film produced by achemical vapor deposition (CVD) method, moisture resistance can beimproved. In such a case, when the underlying insulating film is madefrom a conventional polyimide, there take place cracks on the surfaceinorganic film. Therefore, the underlying insulating film should be madefrom the polyimide of this invention.

FIG. 4 is a cross-sectional view of a memory element having an α-raysshielding layer. Numeral 7 denotes a wiring layer and 8 a lead wire.When the low thermal expansion polyimide 9 is used as α-rays shieldinglayer, since differences in thermal expansion coefficients among asilicon wafer 1 and the wiring layer 7 are very small, there takes placeno cracks nor peeling due to thermal stress as in the case of using aconventional polymer, and there arises no problem of lowering inresolving power at the time of pattening of a photoresist due to warpageof the wafer. Further, since the polyimide of this invention isremarkably excellent in heat resistance compared with conventionalpolyimides, it is suitable for use in glass encapsulated semiconductiorelement. The pyrolysis rate of the polyimide of this invention is about1/10 of that of conventional ones.

When the low thermal expansion polyimide of this invention is used asinsulating film for LSI shown in FIGS. 3 and 4, the shrinkage at thetime of curing is prevented by the silicon wafer to show low thermalexpansion.

FIG. 5 shows a cross-sectional view of a metal core wiring board (ametal-plate-base printed wiring board) mounting a film-carrier type LSI.Numeral 10 denotes a metal substrate, 11 a LSI chip produced by afilm-carrier method, 12 a carrier film using the low thermal expansionpolyimide, and 13 a terminal. Since the carrier film 12 is made from thelow thermal expansion polyimide, there can be obtained LSI 11 with highaccuracy and high density. Further, since the stress applied to a solderball 14 for bonding is reduced remarkably, breaking as a result offatigue is also reduced. Further, by applying the low thermal expansionpolyimide to an insulating film 16 at a wiring portion 15 formed on themetal substrate 10, there can be obtain a printed wiring board withoutcrook, which results in making high accuracy high density assemblypossible.

FIG. 6 is a cross-sectional view of a metal-plate-based printed circuitmounting a LSI bonded by lead wire. Numeral 17 denotes a LSI chipobtained by a leadwire bonding method. Other numerals are the same asthose in FIG. 5, etc.

The low thermal expansion polyimide of this invention can effectively beused in the following application, although not particularly shown in adrawing. When a metal foil such as stainless steel foil coated with athin film of the low thermal expansion polyimide is used as a substratedin a solar cell using amorphous silicon, the generation of cracks on theamorphous silicon thin film is reduced remarkably compared with the caseof using conventional polymimdes.

When the low thermal expansion polyimide is used as matrix resin forfiber reinforced laminates, not only the thermal expansion coefficientat the direction along the layers by the fiber reinforcement but alsothat at the flatwise direction, that is, perpendicular to the directionalong the layers, can be reduced. Further, since the difference inthermal expansion coefficients between the fiber material and thepolyimide is small, there arises no local thermal stress nor interfacialpeeling due to heat shock, and the like.

When the low thermal expansion polyimide of this invention is used as amolding material, even if an insert made of a metal or ceramic is moldedtogether, there takes place no problem of generation of cracks, cracksand deformation of the insert per se.

When the low thermal expansion polyimide is used in the form of fiber,for example, as reinforcing fibers for FRP, the weight-saving is moreremarkable than the use of glass fiber. Further, when it is used asmulti-layer wiring board for computors, the dielectric constant becomessmaller compared with the case of using glass fiber.

The composite shaped article of this invention can be shaped into onebody directly or by using an adhesive or binder.

This invention is illustrated by way of the following Examples.

Examples 1 to 8, Comparative Examples 1 to 16

In a four-necked flask equipped with a thermometer, a stirringapparatus, a reflux condenser, and a nitrogen introducing tube, adiamine was placed in an amount as shown in Table 1 and dissolved with850 g of N-methyl-2-pyrrolidone (NMP). Then, the flask was dipped in awater bath at 0° to 50° C. and a tetracarboxylic acid dianhydride asshown in Table 1 was added thereto while controlling the exothermicheat. After dissolving the tetracarboxylic dianhydride, the water bathwas removed and the reaction was continued for about 5 hours at nearroom temperature to give a polyamide-acid varnish. When the viscosity ofvarnish became too high, stirring with heating (cooking) at 80° to 85°C. was continued until the viscosity became 50 poises at 25° C.

The thermal expansion coefficient of polyimide obtained by heating apolyamide-acid was measured as follows. A varnish was coated uniformlyon a glass plate by using an applicator and dried at 80° to 100° C. for30 to 60 minutes to make a film-like form. Then, the film was strippedoff from the glass plate and fixed on an iron frame (bifix cure) or hungby using a spring to allow free shrinkage (free cure), and held at 200°C., 300° C. or 400° C. for 60 minutes to give a polyimide film of 20 to200 μm thick. A film of 3 mm×80 mm was cut off and placed between twoglass plates, and heated at 400° C. again, followed by gradual coolingto remove residual strain. Then, dimensional change was measured underconditions of 5° C./min by using a thermal mechanical analyzer and thethermal expansion coefficient was obtained from the dimensional changeamount below the glass transition temperature. As mentioned above,accurate thermal expansion coefficient cannot be meadured, unlessabsorbed moisture or solvent in film or residual strain by imidizationis removed sufficiently and the imidization reaction is substantiallycompleted. If an absorbed moisture is present, the thermal expansioncoefficient of the film is apparently measured with a lower value at atemperature range of room temperature to 150° C. due to removal ofmoisture. Further, if the residual strain is retained or the imidizationreaction is is not completed, the linear expansion coefficient issometimes apparently measured with a lower value due to the removal ofresidual strain near the glass transition temperature (Tg) or shrinkageby dehydration by imidization reaction. Further, when the film is fixedon the iron frame and cured and broken during the curing, the thermalexpansion coefficient is measured with a rather higher value due toinsufficient orientation treatment. These should be taken intoconsideration.

Table 1 shows thermal expansion coefficients of polyimide films afterbifix cured and free cured. Further, the data on Table 1 are plotted inFIG. 7 taking the linear expansion coefficients by free cure ofpolyimide along the abscissa axis and the linear expansion coefficientsby bifix cure of polyimide along the ordinate axis. As is clear fromFIG. 7, the polyimides of this invention clearly have very low thermalexpansion coefficients compared with conventional polyimides.

From the viewpoint of chemical structure, the low thermal expansion canbe obtained when P-PDA, DATP, o-TOLID, and p-DATOL are used as diamine,and PMDA and BPDA are used as tetracarboxylic dianhydride. In otherwords, the following conditions can be summarized for giving the lowthermal expansion coefficients:

(1) the main skelton has a benzene ring and an imide ring,

(2) the bonding of the benzene ring is by the parabonding, and

(3) the benzene ring can contain a substituent such as a methyl group atside chains.

                                      TABLE 1                                     __________________________________________________________________________                                 Linear expansion                                                    Tetracarboxylic                                                                         coefficient ×                                     Diamine     acid dianhydride                                                                        10.sup.-5 K.sup.-1                               Example No.                                                                          Name  Amount (g)                                                                          Name                                                                              Amount (g)                                                                          Bifix cure                                                                          Free cure                                                                           Note                                 __________________________________________________________________________    Example 1                                                                            p-PDA 49.72 PMDA                                                                              100.3 --    --    Brittle                              Example 2                                                                             "    40.31 BPDA                                                                              109.7 0.9   2.1                                        Comparative                                                                           "    37.69 BTDA                                                                              112.3 2.3   4.7                                        Example 1                                                                     Example 3                                                                            DATP  81.62 PMDA                                                                              68.38 0.6   1.0                                        Example 4                                                                             "    70.42 BPDA                                                                              79.58 0.6   1.5                                        Comparative                                                                           "    67.03 BTDA                                                                              82.97 2.1   3.3                                        Example 2                                                                     Example 5                                                                            o-TOLID                                                                             75.03 PMDA                                                                              74.97 0.3   0.8                                        Example 6                                                                             "    63.89 BPDA                                                                              86.11 0.6   2.8                                        Comparative                                                                           "    60.58 BTDA                                                                              89.42 1.9   5.8                                        Example 3                                                                     Example 7                                                                            p-DATOL                                                                             53.85 PMDA                                                                              96.15  0.04 1.6                                        Example 8                                                                             "    44.01 BPDA                                                                              106.0 0.8   1.9                                        Comparative                                                                          p-DATOL                                                                             41.24 BTDA                                                                              108.8 2.8   5.0                                        Example 4                                                                     Comparative                                                                          m-DATOL                                                                             53.85 PMDA                                                                              96.15 --    --                                         Example 5                                                                     Comparative                                                                           "    44.01 BPDA                                                                              106.0 3.2   5.0                                        Example 6                                                                     Comparative                                                                           "    41.24 BTDA                                                                              108.8 3.5   5.1                                        Example 7                                                                     Comparative                                                                          m-PDA 49.72 PMDA                                                                              100.3 3.9   4.0                                        Example 8                                                                     Comparative                                                                           "    40.31 BPDA                                                                              109.7 4.0   4.2                                        Example 9                                                                     Comparative                                                                           "    37.69 BTDA                                                                              112.3 3.1   3.9                                        Example 10                                                                    Comparative                                                                          DDE   71.79 PMDA                                                                              78.21  2.36  5.13                                      Example 11                                                                    Comparative                                                                           "    60.75 BPDA                                                                              89.25  4.32  5.20                                      Example 12                                                                    Comparative                                                                           "    57.49 BTDA                                                                              92.51  4.28  5.52                                      Example 13                                                                    Comparative                                                                          DDS   74.69 PMDA                                                                              75.31  4.84  6.03                                      Example 14                                                                    Comparative                                                                           "    74.69 BPDA                                                                              86.45  4.91  5.85                                      Example 15                                                                    Comparative                                                                           "    60.25 BTDA                                                                              89.75  5.62  5.78                                      Example 16                                                                    Comparative                                                                          DDM   71.42 PMDA                                                                              78.58  4.20  4.50                                      Example 17                                                                    Comparative                                                                           "    60.39 BPDA                                                                              89.61  4.18  5.03                                      Example 18                                                                    Comparative                                                                           "    57.14 BTDA                                                                              92.86  4.50  5.00                                      Example 19                                                                    Comparative                                                                          BAPB  94.22 PMDA                                                                              55.78  5.33  6.32                                      Example 20                                                                    Comparative                                                                           "    83.4  BPDA                                                                              66.6   5.32  6.17                                      Example 21                                                                    Comparative                                                                           "    80.02 BTDA                                                                              69.98  4.30  4.37                                      Example 22                                                                    Comparative                                                                          DAPP  97.95 PMDA                                                                              52.05  5.11  6.57                                      Example 23                                                                    Comparative                                                                           "    87.38 BPDA                                                                              62.62  5.70  5.80                                      Example 24                                                                    Comparative                                                                           "    84.04 BTDA                                                                              65.96  5.39  5.59                                      Example 25                                                                    Comparative                                                                          DAPFP 105.6 PMDA                                                                              44.42  4.75  6.37                                      Example 26                                                                    Comparative                                                                           "    95.69 BPDA                                                                              54.31  5.23  6.15                                      Example 27                                                                    Comparative                                                                           "    92.51 BTDA                                                                              57.49  5.47  6.24                                      Example 28                                                                    __________________________________________________________________________     (Note)                                                                        pPDA = pphenylenediamine                                                      DATP = 4,4diaminoterphenyl                                                    oTOLID = otolidine                                                            pDATOL = 2,5diaminotoluene                                                    mDATOL = 2,4diaminotoluene                                                    mPDA = mphenylenediamine                                                      DDE = 4,4diamiono ether                                                       DDS = 4,4diamino sulfide                                                      DDM = 4,4diamino diphenyl methane                                             BAPB = 4,4bis(4-aminophenoxy)biphenyl                                         DAPP = 2,2bis{4(4-aminophenoxy)phenyl}propane                                 DAPFP = 2,2bis{4(4-aminophenoxy)phenyl}hexafluoropropane                      DMDA = pyromellitic dianhydride                                               BPDA = 3,3',4,4biphenyltetracarboxylic dianhydride                            BTDA = 3,3',4,4benzophenonetetracarboxylic dianhydride                   

Examples 9 to 14

The low thermal expansion polymides of this invention can be blended orcopolymerized with conventional polyimides having larger thermalexpansion coefficients or with conventional polyamide-acid to controlthe thermal expansion coefficients properly.

Thermal expansion coefficients of copolymerized or blended materials areshown in Table 2. As is clear from Table 2, there is almost linearrelation between the copolymerization or blending ratio and the thermalexpansion coefficient. This means that the control can be conductedeasily.

                  TABLE 2                                                         ______________________________________                                                         Linear expansion                                                              coefficient ×                                                           10.sup.-5 K.sup.-1                                           Example                                                                              Mixing proportion (g)                                                                         Bifix    Free                                          No.    p-PDA   DDE     BPDA  cure   cure Note                                 ______________________________________                                        Example                                                                              34.83   21.5    93.67 3.3    1.7  Copolymer                            Example                                                                              21.78   40.34   87.88 4.1    3.0   "                                   10                                                                            Example                                                                              10.26   56.98   82.76 5.6    4.3   "                                   11                                                                            Example                                                                              34.83   21.5    93.67 3.0    1.5  Blend of                             12                                       Example 1                                                                     and Com-                                                                      parative                                                                      Example                                                                       12                                   Example                                                                              21.78   40.34   87.88 3.7    2.6  Blend of                             13                                       Example 1                                                                     and Com-                                                                      parative                                                                      Example                                                                       12                                   Example                                                                              10.26   56.98   82.76 4.5    3.8  Blend of                             14                                       Example 1                                                                     and Com-                                                                      parative                                                                      Example                                                                       12                                   ______________________________________                                    

Example 15 (Examination of Properties)

(1) Samples

A polyamide-acid varnish was prepared by reacting an aromatic diamine aslisted in Table 3 with an aromatic tetracarboxylic dianhydride as listedin Table 3 in N-methyl-2-pyrrolidone at room temperature. The varnishwas coated on a glass plate and dried at 100° C. for 1 hour. Then, theresulting film was stripped off and cured at 200° C. for 1 hour and at400° C. for 1 hour in a nitrogen gas to give a polyimide film. It wasalso found that behavior for thermal expansion changed when curingshrinkage was conducted freely (free cure) or shrinkage was prevented byfixing on an iron frame at one direction (unifix cure) or at twodirections (bifix cure).

                                      TABLE 3                                     __________________________________________________________________________     ##STR7##                                                                      R.sub.1                                                                                                          ##STR8##                                  __________________________________________________________________________    (A)                                                                               ##STR9##                       (1)                                                                              ##STR10##                                  (p-PDA)                           (PMDA)                                   (B)                                                                               ##STR11##                      (2)                                                                              ##STR12##                                  (m-PDA)                           (BPDA)                                   (C)                                                                               ##STR13##                      (3)                                                                              ##STR14##                                  (o-TOLID)                         (BTDA)                                   (D)                                                                               ##STR15##                                                                    (DATP)                                                                     (E)                                                                               ##STR16##                                                                    (DDE)                                                                      (F)                                                                               ##STR17##                                                                    (DDS)                                                                      (G)                                                                               ##STR18##                                                                    (DDM)                                                                      (H)                                                                               ##STR19##                                                                    (m-DATOL)                                                                  (I)                                                                               ##STR20##                                                                    (BAPB)                                                                     (J)                                                                               ##STR21##                                                                    (DAPP)                                                                     (K)                                                                               ##STR22##                                                                    (DAPFP)                                                                    __________________________________________________________________________     Note                                                                          A resin number is determined by a combination of (A) to (K) as R.sub.1 an     (1) to (3) as R.sub.2.                                                        For example, No. A2 means a polyimide having the structure of (A) as          R.sub.1 and (2) as R.sub.2.                                              

(2) Coefficients of thermal expansion

A film of 5 mm wide, 65 mm long (between chucks) and 20 to 100 μm thickwas used as sample. The sample was applied to a thermal mechanicalanalyzer (TMA 1500, mfd. by Shinku Riko K.K.) and measured under theconditions of 5° C./min. Since the thermal expansion coefficient hadtemperature-dependency, an average value between 50° to 250° C. was usedas a representative value.

(3) Wide angle X-ray diffraction

A sample of 30 mm wide, 40 mm long and about 40 μm thick was measured byusing a Geiger-Flex RAD (mfd. by Rigaku Denki Co., Ltd.)

(4) Results and Discussions

It was found that behavior of thermal expansion for polyimides changedvariously whether curing shrinkage in the course of heat cure ofpolyamide-acid film to polyimide was prevented or not. FIG. 8 showsbehavior of thermal expansion for polyimide No. D1 obtained from DATPand PMDA. The thermal expansion coefficient of conventional polyimide isusually 4 to 6×10⁻⁴ K⁻¹ and there is a dimensional change of 0.8 to 1.2%with the temperature change of from 50° to 250° C. In contrast, thepolyimide of No. D1 shows dimensional change of about 0.2% (free cure)or about 0.1% (bifix cure) in the temperature range of 50° to 250° C.

FIG. 9 also shows behavior of thermal expansion for polyimide No. A2.The dimensional change by the bifix cure is 0.1% or less, while that byfree cure becomes as large as about 0.4% in the range of 50° to 250° C.with a relative increase of thermal expansion coefficient. Using thissample (No. A2), prevention of curing shrinkage at only one axis wasalso studied. As a result, it was found that at the prevented direction(UFX-X) the thermal expansion coefficient became smaller and thedimensional change was almost 0% up to near 250° C. But, at thedirection perpendicular to the controlled direction (UFX-Y), thebehavior was almost the same as the case of free cure. That is, by amethod for giving a slight curing shrinkage (linear shrinkage, about10%) at the time of imidization, the resulting effect was the same asthat obtained by stretching a conventional prepolymer several to severaltens times.

Similarly, behavior of thermal expansion for Nos. A3 and F3 (these beingconventional polyimides) is also shown in FIGS. 10 and 11.

FIG. 12 shows a relationship between the chemical structures and thermalexpansion coefficients.

From the above results, the kinds of polyimides can roughly be dividedinto the following 4 groups.

○1 Polyimides having small thermal expansion coefficients both in freecure and bifix cure. This group includes those obtained from acombination of o-TOLID or DATP, and PMDA.

○2 Polyimides having slightly large thermal expansion coefficients inthe case of free cure but having about half values in the case of bifixcure. This group includes those obtained from a combination of BPDA andp-PDA, o-TOLID or DATP.

○3 Polyimides having considerably large thermal expansion coefficientsin the case of free cure but having about half values in the case ofbifix cure. This group includes those obtained from a combination ofp-PDA, o-TOLID, DATP, or m-PDA, and BTDA, a combination of DDE and PMDA,or a combination of 2,4-DATO and BPDA.

○4 Polyimides having large thermal expansion coefficients both in freecure and bifix cure. Almost all polyimides other than those mentionedabove belong to this group.

On the other hand, it is well known that thermal expansion coefficientsof polymers become smaller by crystallization caused by stretching.Crystallizability of polyimides was studied by using wide angle X-raydiffraction. Measured results were shown in FIGS. 13 and 14. FIG. 13shows that No. D1 belonging to the group ○1 has sharp reflection andthus a crystallizability. Similarly, No. C1 seems to havecrystallizability to some extent. In contrast to them, Nos. A2 of thegroup ○2 and No. A3 of the group ○3 have slight orientation comparedwith No. F3 of the group ○4 , but they seem to have no crystallizabilityin contrast to Nos. D1 and C1.

In FIG. 14, differences due to free cure, bifix cure, UFX-X and UFX-Ywere studied as to No. A2, but there is almost no difference in wideangle X-ray diffraction data. If evaluated in detail, there is atendency that the height of the curve is lower in the case of free cure.From these results, there is a clear tendency that the strongercrystallizability is, the smaller the thermal expansion coefficientbecomes. But the controlling effect on curing shrinkage cannot beexplained sufficiently by this only. The degree of order in more longerdistance seems to be also important.

Next place, conformations of polyimides belonging to individual groups○1 to ○4 were studies. As shown shown in FIG. 15, it can be admittedsome community to some extent. The polyimides belonging to the group ○1can only take conformation wherein both the diamine and tetracarboxylicacid components lie linearly on almost the same plane as shown in FIG.15 (I). Therefore, they are rod-like at the direction of molecular axisand rigid, as well as liable to be crystallized. The polyimidesbelonging to the group ○2 have considerably linear conformation,although take a somewhat zigzag structure due to the biphenyl of BPDA asshown in FIG. 15 (II). The polyimides belonging to the group ○3 canfurther be divided into two groups; one of which is formed from p-PDA,o-TOLID or DATP and BTDA and lies in almost the same plane as shown inFIG. 15 (III) but is hardly arranged on a straight line depending onbenzophenone skeltons, and another of which turns at the ether bonds asshown in FIG. 15 (IV) and does not lie on the same plane but the skeltonbetween two ether bonds lies linearly. The polyimides obtained from acombination of PMDA and DDM or DDS also belong to the same type. Almostall the other polyimides lose regularity by ether, thioether, methylene,ketone, or the like as shown in FIG. 15 (V).

What is claimed is:
 1. A composite shaped article comprising aninorganic material bonded with a low thermal expansion resin materialshaped into one body, said resin material comprising a polyimide havingas chemical structural unit at least one aromatic ring which can rotatearound its molecular axis but has no flexibility at another direction,said polyimide having a molecular chain oriented so as to have asubstantially linear structure on a plane by stretching a film, fiber orcoating of the polyimide or a precursor thereof uniaxially or biaxiallywith a stretcher and exhibiting a linear thermal coefficient of 3×10⁻⁵to 4×10⁻⁷ K⁻¹.
 2. A composite shaped article according to claim 1, whichis a flexible printed circuit obtained by forming metallic wiring on asubstrate made from the low thermal expansion resin material.
 3. Acomposite shaped article according to claim 1, which is a large-scaleintegrated circuit (LSI) having a wiring insulating film made from thelow thermal expansion resin material between a silicon wafer and wiring,or between wiring and wiring.
 4. A composite shaped article according toclaim 1, which is a large-scale integrated circuit wherein a siliconwafer forming wiring thereon is covered with the low thermal expansionresin material.
 5. A composite shaped article according to claim 1,wherein the polyimide has a main chain of at least one of the formulae:##STR23## wherein R is an alkyl group, a fluorinated alkyl group, analkoxy group, a fluorinated alkoxy group, an acyl group, or a halogenatom; l is zero or an integer of 1 to 4; m is zero or an integer of 1 to2; and n is zero or an integer of 1 to
 3. 6. A composite shaped articleaccording to claim 1, wherein the polyimide has as repeating unit atleast one member selected from the group consisting of ##STR24## whereinR is an alkyl group, a fluorinated alkyl group, an alkoxy group, afluorinated alkoxy group, an acyl group, or a halogen atom; l is zero oran integer of 1 to 4; m is zero or an integer of 1 to 2; n is zero or aninteger of 1 to 3; and k is zero or an integer of 1 to
 3. 7. A compositeshaped article according to claim 1, wherein the polyimide iscopolymerized or blended with another polymer.
 8. A composite shapedarticle according to claim 1, wherein the polyimide is obtained bypolymerization of an aromatic aminodicarboxylic acid derivative, orobtained by reacting an aromatic diamine or aromatic diisocyanate withan aromatic tetracarboxylic acid derivative.
 9. A composite shapedarticle according to claim 7, wherein the another polymer is a polyimideobtained from a diamine and a tetracarboxylic acid derivative.
 10. In amulti-layer wiring board comprising layers of a wiring insulating filmand a wiring, the improvement wherein the wiring insulating film is madefrom a low thermal expansion resin material comprising a polyimidehaving as a chemical structural unit at least one aromatic ring whichcan rotate around its molecular axis but which has no flexibility inaother direction and which is bonded at para positions, said polyimidehaving a molecular chain oriented so as to have a substantially linearstructure on a plane by stretching a film of the polyimide or aprecursor thereof uniaxially or biaxially with a stretcher andexhibiting a linear thermal expansion coefficient of 3×10⁻⁵ to 4×10⁻⁷k⁻¹.
 11. A multi-layer wiring board according to claim 10, wherein thepolyimide has as repeating units at least one member selected from thegroup consisting of: ##STR25## wherein R is an alkyl group, afluorinated alkyl group, an alkoxy group, a fluorinated alkoxy group, anacyl group or a halogen atom; l is zero or an integer of 1 to 4; m iszero or an integer of 1 to 2; n is zero or an integer of 1 to 3; and kis zero or an integer of 1 to
 3. 12. A multi-layer wiring boardaccording to claim 10, wherein the polyimide has as repeating units atleast one member selected from the group consisting of: ##STR26##wherein R is an alkyl group, a fluorinated alkyl group, an alkoxy group,a fluorinated alkoxy group, an acyl group or a halogen atom; l is zeroor an integer of 1 to 4; m is zero or an integer of 1 to 2; n is zero oran integer of 1 to 3; and k is zero or an integer of 1 to
 3. 13. In aflexible printed circuit board comprising a substrate and a metallicwiring formed on the substrate, the improvement wherein the substrate ismade from a low thermal expansion resin material comprising a polyimidehaving as a chemical structural unit at least one aromatic ring whichcan rotate around its molecular axis but which has no flexibility inanother direction and which is bonded at para positions, said polyimidehaving a molecular chain oriented so as to have a substantially linearstructure on a plane by stretching a film of the polyimide or aprecursor thereof uniaxially or biaxially with a stretcher andexhibiting a linear thermal expansion coefficient of 3×10⁻⁵ to 4×10⁻⁷k⁻¹.
 14. A flexible printed circuit board according to claim 13, whereinthe polyimide has as repeating units at least one member selected fromthe groups consisting of: ##STR27## wherein R is an alkyl group, afluorinated alkyl group, an alkoxy group, a fluorinated alkoxy group, anacyl group or a halogen atom; l is zero or an integer of 1 to 4; m iszero or an integer of 1 to 2; n is zero or an integer of 1 to 3; and kis zero or an integer of 1 to
 3. 15. A flexible printed circuit boardaccording to claim 13, wherein the polyimide has as repeating units atleast one member selected from the group consisting of: ##STR28##wherein R is an alkyl group, a fluorinated alkyl group, an alkoxy group,a fluorinated alkoxy group, an acyl group or a halogen atom; l is zeroor an integer of 1 to 4; m is zero or an integer of 1 to 2; n is zero oran integer of 1 to 3; and k is zero or an integer of 1 to
 3. 16. In afloppy disk comprising a base plate and an information recording layerformed on the base plate, the improvement wherein the base plate is madefrom a low thermal expansion resin material comprising a polyimidehaving as a chemical structural unit at least one aromatic ring whichcan rotate around its molecular axis but which has no flexibility inanother direction and which is bonded at para positions, said polyimidehaving a molecular chain oriented so as to have a substantially linearstructure on a plane by stretching a film of the polyimide or aprecursor thereof uniaxially or biaxially with a stretcher andexhibiting a linear thermal expansion coefficient of 3×10⁻⁵ to 4×10⁻⁷k⁻¹.
 17. A floppy disk according to claim 16, wherein the polyimide hasas repeating units at least one member selected from the groupconsisting of: ##STR29## wherein R is an alkyl group, a fluorinatedalkyl group, an alkoxy group, a fluorinated alkoxy group, an acyl groupor a halogen atom; l is zero or an integer of 1 to 4; m is zero or aninteger of 1 to 2; n is zero or an integer of 1 to 3; and k is zero oran integer of 1 to
 3. 18. A floppy disk according to claim 16, whereinthe polyimide has as repeating units at least one member selected fromthe group consisting of: ##STR30## wherein R is an alkyl group, afluorinated alkyl group, an alkoxy group, a fluorinated alkoxy group, anacyl group or a halogen atom; l is zero or an integer of 1 to 4; m iszero or an integer of 1 to 2; n is zero or an integer of 1 to 3; and kis zero or an integer of 1 to
 3. 19. A memory element comprising asemiconductor element and an alpha-ray shielding layer coated on saidsemiconductor element, said alpha-rays shield layer being made from alow thermal expansion resin material comprising a polyimide having as achemical structural unit at least one aromatic ring which can rotatearound its molecular axis but which has no flexibility in anotherdirection and which is bonded at para positions, said polyimide having amolecular chain oriented so as to have a substantially linear structureon a plane by stretching a film of the polyimide or a precursor thereofuniaxially or biaxially with a stretcher and exhibiting a linear thermalexpansion coefficient of 3×10⁻⁵ to 4×10⁻⁷ k⁻¹.
 20. A memory elementaccording to claim 19, wherein the polyimide has as repeating units atleast one member selected from the group consisting of: ##STR31##wherein R is an alkyl group, a fluorinated alkyl group, an alkoxy group,a fluorinated alkoxy group, an acyl group or a halogen atom; l is zeroor an integer of 1 to 4; m is zero or an integer of 1 to 2; n is zero oran integer of 1 to 3; and k is zero or an integer of 1 to
 3. 21. Amemory element according to claim 19, wherein the polyimide has asrepeating units at least one member selected from the group consistingof: ##STR32## wherein R is an alkyl group, a fluorinated alkyl group, analkoxy group, a fluorinated alkoxy group, an acyl group or a halogenatom; l is zero or an integer of 1 to 4; m is zero or an integer of 1 to2; n is zero or an integer of 1 to 3; and k is zero or an integer of 1to 3, and exhibiting a linear thermal expansion coefficient of 3×10⁻⁵ to4×10⁻⁷ k⁻¹.